The Hybrid Fiber-Coax (HFC) network has continuously evolved to deploy fiber deeper. It is foreseen that in the near future, it will become a Fiber to the Premise (FTTP) or Fiber to the Home (FTTH) network. One of the technologies in consideration to allow this transition is Radio Frequency over Glass (RFoG). RFoG is a deep-fiber network design in which the coax portion of the hybrid fiber coax (HFC) network is replaced by a single-fiber passive optical network (PON). in order to deliver cable services through the passive optical network (PON) style FTTP network infrastructure .PON is a telecommunications technology that implements a point-to-multipoint architecture, in which unpowered fiber optic splitters are used to enable a single optical fiber to serve multiple end-points such as customers, without having to provision individual fibers between the hub and customer. The RFoG system is defined to begin where the network becomes passive, extending from that point to the customer premise (CPE). This interface is referred to as the optical node. The CPE is a service provider equipment that is located on the customer's premises (physical location) rather than on the provider's premises or in between. Some examples of the CPE include, but not limited telephone handsets, cable TV set-top boxes, and digital subscriber line routers. The RFoG system is defined to terminate at the subscriber-side interface of an RFoG Optical Network Unit (R-ONU) or customer premise equipment (CPE) at the home. An optical network is a means of communication that uses signals encoded onto light to transmit information among various nodes of a telecommunications network.
As part of the continuous effort to converge towards next-generation networks (NGN), the RFoG has been designed to share trunk fibers with the traditional Passive Optical Network (PON). A RFoG implementation that supports the data over cable services interface specification (DOCSIS) RF infrastructure along with traditional PON has been considered as the truly Hybrid PON (HPON) architecture. HPON enables other fiber deep technologies such as Fiber to the Curb (FTTC), Fiber to the Multiple Dwelling Unit (FTT-MDU), and fiber to the deep node (N+0). As HPON becomes popular as a network capable to provide higher amounts of bandwidth than legacy HFC, and as it becomes commercially attractive for operators, the increasing demand of network coverage is driving new requirements for key enabling products. It is the case when the fiber links from the hubs to the optical nodes are very long distances. Even when some operators were careful to deploy enough nodes to assure that the vast majority of the fiber distances were 20 km or less (e.g. PON services). Other operators allowed much longer fiber distances (˜60 km) to be used (e.g. RFoG). The long fiber distances in these HPON networks negatively impacts the available performance budget and makes it difficult to further increase network coverage. In addition, there are other difficulties such as the limited number of trunk fibers, and the wavelength congestion at each fiber. For the latter, it is worth noting that HPON networks are required to operate at specific wavelengths for a Downstream (DS) and an Upstream (US) in order to allow coexistence between PON and RFoG services. Therefore, it is not possible to operate more than one service (RFoG+PON) group on the same trunk fiber at the same time.
In case of PON systems, in order to accommodate the long distances and limited numbers of fibers, many operators are deploying PON extenders. The PON extender is an active device in the node that regenerates the electrical signals so that they can be retransmitted to the local serving area with high fidelity, eliminating the penalty of long link budgets between hubs and nodes. On the other hand, for RFoG services, EP patent publication number EP1235434 B1 describes a “dual broadband optoelectronic repeater” which makes it possible to use the network topology of the point-to-point type from the node to each individual subscriber to extend the performance budget. The EP 1235434 proposes to eliminate branches between the CPE and the final optical transmitter or receiver in the dual broadband optoelectronic repeater. However, the aforementioned repeater presents several disadvantages in terms of cost effectiveness, power consumption, coverage and capacity. In the forward (a.k.a. downstream) path, receiving, amplifying and retransmission of DS signals with one laser per subscriber increases considerable cost and power dissipation. In addition, since the repeater is proposed at the node location, it still presents link budget limitation when the network coverage increase is required further away from the node's service area.
An example of a RFoG architecture 100 is illustrated in FIG. 1 when a network coverage extension is needed from a first service area 108 to another new remote service area (second service area) 110. An optical node 104 depicted in FIG. 1 is an active RFoG node. The RFoG node is used for fiber deep applications, where service areas (first service are 108 and second service area 110) are connected via optical fiber to the node. In the implementation, as shown in FIG. 1, the conversion of optical/electrical for DS signals and the conversion of electrical/optical for US signals occur at the service area location in the MDU. The optical node 104 is optically coupled to a head end 102. A head end includes multiple devices for delivery of video and data services including EdgeQAMS (EQAMs) for video, cable modem termination systems (CMTS) for data, and other processing devices for control and management. These systems are connected to multiple fiber optic cables that go to various neighborhood locations. A fiber optic neighborhood node is located between each fiber optic cable and a corresponding trunk cable which in turn is interconnected to the homes through branch networks and feeder cables. Because the trunk cable, as well as the branch networks and feeder cables, each propagate RF signals using coaxial cable, HFC nodes (preferably located in the MDU) are used to convert the optical signals to electrical signals that can be transmitted through a coaxial medium, i.e. copper wire to be distributed at homes. The optical node 104 of FIG. 1, as discussed above is an active RFoG node transmits and receives optical signals from the head end 102 and to MDUs. Similarly, when electrical signals from the home reach the node over the coaxial medium, those signals are converted to optical signals and transmitted across the fiber optic cables back to the systems at the head end. Accordingly, a head end is a control system that receives RF signals for processing and distribution over a cable television system. Specifically, the head end receives the RF signals containing data signals, multiplexes them using a RF combining network, converts the combined RF signal to an optical signal and outputs the output signal. The combined and converted RF signals comprise a downstream (DS) signal, which refers to a signal transmitted from the head end 102 to the optical node 104 to an end user via a network (example, coaxial network, local network, etc.). In the upstream (US) direction, the combined and converted RF signals comprise a (US) signal, which refers to a signal transmitted from the end user to the head end 102. The optical node 104 functions to extract the traditional cable signals such as the DS signal having the wavelength of 1550 nm and the US signal having the wavelength of 1610 nm. The traditional cable signals are processed to be sent to the first service area 108 and to the second service area 110. The optical node 104 is also coupled to a PON unit 106 to provide cable services to PON network (not shown). Some examples of a PON network include Broadband PON, Gigabit PON, Ethernet PON. In the scenario presented in FIG. 1, it is assumed that a multiple dwelling unit (MDU) in the first service area 108 contains multiple MDU CPEs 112a-112n. However, network coverage increase can apply in other scenarios, such as but not limited to service areas where each user has direct fiber connection to home with independent CPEs connected directly to the optical node 104. As shown in FIG. 1, is an RFoG network coverage to the first service area 108 (distance=FLn−1(a)). A typical way to increase coverage in the RFoG network, is the introduction of expensive low-distortion optical amplifiers, extra optical splitting, and external dispersion compensation modules at the optical node 106 to achieve a good quality signal at longer distances in order to service additional remote areas (distance=FLn−1(a)+FLn−1 (b) from FIG. 1). However, this implementation only provides budget performance extension for the forward or the downstream (DS), and the return or upstream (US) still suffers poor signal quality. On the other hand, an extra optical fiber from the optical node 104 to the second service area 110 is required. Therefore, in most of the cases, it may be preferable to install another remote node at a location closer to the second service area 110 in order to increase network coverage. This approach includes the necessity to install new fiber links all the way down from the Head End 102 to the new node and to a MDU CPE 114 in the second service area 110.
FIGS. 2A and 2B illustrate a schematic diagram of conventional MDU CPE devices 200 and 240 respectively typically used by the cable operators to provide both traditional cable service and PON service on RFoG system expanded to support PON architecture and services. Specifically, FIG. 2A depicts a conventional MDU CPE device 200 over RFoG architecture with an option to upgrade to a PON optical network unit (ONU) outside of the MDU CPE and FIG. 2B depicts a conventional MDU CPE device 240 over RFoG architecture including the PON ONU integrated into the same MDU CPE device. In one example, there is shown an RFoG wavelength division multiplexer (WDM) filter 204 optically coupled to a head end 202. As discussed above, a Head End is a control system that receives data (such as television, internet, voice etc.) signals for processing and distribution over a cable television system. The RFoG WDM filter 204 functions similar to the optical node 104 of FIG. 1 such that the first optical filter 104 extracts the traditional cable signals (DS optical signal having the wavelength of 1550 nm and US optical signal having the wavelength of 1610 nm). As such, the RFoG WDM filter 204 functions to separate the DS optical signal from the US optical signal. The RFoG WDM filter 204 transmits the DS optical signal to a first RFoG optical receiver (RFoG optical RX1) 206, which converts the DS optical signal into RF domain into a DSRF signal containing data. The RFoG optical RX1 206 sends the DSRF signal to a diplexer 210. Data may flow not only from the head end 102 to a MDU coaxial network 212 to reach the various neighborhoods, but also from the MDU coaxial network 212 to the head end 102, In order to provide this functionality, typically one spectrum of frequencies are dedicated to deliver forward (DS) path signals from the head end 102 to a MDU coaxial network 212 to reach the various neighborhood locations and another (typically much smaller) spectrum of frequencies are dedicated to deliver return (US) path signals from the MDU coaxial network 212 to the head end 102. The diplexer 210 provides such functionality. Specifically, the diplexer 210 includes two or more band pass filters to separate the forward (Ds) path signals from the return (US) path signals, and separately amplifies the signals from each respective direction in their associated frequency range . . . . The diplexer 210 includes a high frequency filter 210a and a low frequency filter 210b. As such, the DSRF signal is filtered in the high frequency mode of the diplexer 210 and the filtered DSRF signal is outputted to the MDU coaxial network 212 for connection to different users. The diplexer 210 receives the USRF signal from the MDU coaxial network in the low frequency mode and transmits to the analog driver 214. In one example, the analog driver 214 is an RF amplifier which functions to convert a lower power radio frequency signal into a higher power radio frequency signal. As such, the analog driver 214 increases the power of the USRF signal. The analog driver 214 transmits the USRF signal to a first RFoG optical transmitter (RFoG optical TX1) 208, which converts it back into the optical domain to an US optical signal and transmits to the RFoG WDM filter 204 to be transmitted to head end 202. As shown in FIG. 2A is a first link from the RFoG WDM filter 204 to couple with a local PON network (not shown) as an option to upgrade to PON cable services via a PON located outside of the MDU CPE device 200.
FIG. 2B shows a PON extender 241 integrated within the MDU CPE device 240 to extend PON cable services such that one MDU may support both the traditional cable service (Coaxial) and the PON cable service. The PON extender 241 includes a PON optical unit (ONU) 242, a PON WDM filter 244, a first PON optical receiver (PON optical RX1) 246, and a first PON optical transmitter (PON optical TX1) 248 and a digital driver 250. A PON ONU may be a gigabit PON (GPON or gigabit Ethernet PON (GEPON) chip sets. The PON WDM filter 244, extracts the PON services (DS PON optical signal having the wavelength of 1490 nm and US PON optical signal having the wavelength of 1310 nm) received from the head end 202. The PON WDM filter 244 transmits the DS PON optical signal to the PON optical RX1 246, which converts the DS PON optical signal into RF domain into a DSRF PON signal containing data, which is transmitted to the PON ONU 242. The PON ONU 242 is an active device that regenerates the RF signals in order to be retransmitted to a local PON network 252. As such the regenerated DSRF PON signal containing data is outputted from the PON ONU 242 to the local PON network 252 to different users. The PON ONU 242 receives the USRF PON signal via the PON local network 252 and transmits it to the digital driver 250, which functions to convert a lower power RF signal into a higher power RF signal. As such, the digital driver 250 increases the power of the USRF PON signal. The digital driver 250 transmits the USRF PON signal to the RFoG optical TX1248, which converts it back into the optical domain to US optical signal and transmits the US PON optical signal to the PON WDM filter 244 to be transmitted to Head End 202.
The conventional MDU CPEs as described above with respect to FIGS. 2A and 2B are limited to provide RFoG network coverage to one service area and thus do not accommodate the increasing demand of extending the RFoG network coverage to other remote service areas without the need to add more optical fibers and amplifiers (as discussed above with respect to FIG. 1), which results in increase in cost, power consumption, coverage and capacity of the RFoG network.